In typical spot welding, two or more metal sheets are stacked, and two sides of a stacked section of the stacked metal sheets are clamped by a pair of electrodes. A current is then applied to the stacked section to melt a joint interface of the metal sheets through resistance heating, so that the metal sheets are joined together.
There are three major control factors in spot welding: pressure for pressing the electrodes to the metal sheets, a current value, and energization time.
Among them, the current value is a particularly important factor. If the current value is excessively low, a nugget formed after melting has a small diameter, leading to insufficient joint strength. On the other hand, if the current value is excessively high, while the nugget diameter becomes large, splash is generated, and joint strength is disadvantageously fluctuated. An optimum current range in spot welding is therefore a range from a current value above which a certain nugget diameter is secured to a current value below which splash is generated. If such an optimum current range can be expanded, spot welding is facilitated.
Various proposals have been made to expand the optimum current range in spot welding.